How would one calculate Tug of war forces between two vehicles?

Hello! I am working on some calculations for an off-road vehicle on which we are working, and I cannot figure out which figures to use. Really, it's where to start. We have two vehicles (600 lbs. and 550 lbs.) with 10 hp engines that will be hitched together in a tug-of-war type setting. The hitches are attached to 4130 Normalized steel tubing, and I'm trying to calculate how much force is transferred (essentially how beefy we need to build it). I know this isn't a lot of information, but I have a huge brain-block. Any tips to get this started would be so appreciated!

Like many real engineering design applications, you are missing a whole lot of information required for an accurate solution. What we so in this in cases like that is to bracket the solution with a max and min solutions.

For max, assume a coefficient of friction of unity between the tires and ground. Apply that to the smaller vehicle for max draw bar pull. Then throw in a safety factor of 5, to take care of unknowns, the largest of which in this case would be dynamic and shock loads since you are not designing much energy absorption into this system.